Please read the comments - some important stuff may have shown up!
Linux - with GUI
Check that the packet gimp-python is installed.
Copy your .py file into the directory .gimp2.4/plug-ins . This is a hidden directory, you don’t see it in the file browser. With Nautilus you can toggle the display of the hidden files with Ctrl-H.
Then make the file executable by right clicking, selecting “Properties” from the menue and activate “Execute” in the permissions tab.
If GIMP was running, restart it.
Linux - with commandline
Check (with synaptic) that the packet gimp-python is installed.
Enter this:
cp myfile.py ~/.gimp2.4/plug-ins chmod u+x ~/.gimp2.4/plug-ins/myfile.py
If GIMP was running, restart it.
Mac OS
Please write an instruction into the comments - I’ll pull it up to here.
Look through the comments for a solution - but I think there must be a better one.
EDIT by monoceros84: There is a new discussion going on in the forum. Have a look at: http://forum.meetthegimp.org/index.php/topic,227.0
Windows
Copy the files to ~/.gimp2.4/plug-ins
With the following prerequisites:
- PyGTK, PyCairo and PyGobject (all available from the GNOME FTP mirrors as installers) are installed prior to installing GIMP
- ~ is %USERPROFILE%
If GIMP was running, restart it.



